1. Field of the Invention
This invention relates to conductivity sensing systems, for detecting the varying conductivity, for example, of fluids.
2. General Background and State of the Art
In the measurement of the conductivity of fluids, such as distilled or de-ionized water, for example, it is known that the conductivity of the water is a function of impurities or contamination in the fluid or water. Efforts have been made to test the conductivity, but problems have arisen both with respect to short term, or momentary fluctuations of output readings, and also with regard to long term drift of the measured output. In the measurement of the conductivity of fluids, probes or electrodes normally extend into the fluids, and the resistance or conductivity is measured. When the resistance across the probes was employed in one arm of a Wheatstone bridge, and the bridge was energized with direct current, it was determined that (1) the short term variations in the output were so high as to make the determination of conductivity very difficult; and (2) there was a long term drift or shift in the output as a result of metal transfer or precipitation from one electrode to the other.
In order to overcome these problems, it has been determined that excitation with a square wave or alternating current signal with no net direct current flow, avoids the metal transfer and long term drift problems. Further, by using a modified Wheatstone bridge arrangement in which the probes are connected in parallel with resistors in the bridge, the short term fluctuations are substantially stabilized, and accurate output readings may be obtained.
In accordance with one preferred illustrative embodiment of the invention, a stabilized conductivity sensor has a threaded housing for securing into a container for the fluid to be measured, and two conductive metal probes are spaced apart and mounted on the inner portion of the housing. A Wheatstone bridge circuit is also mounted in the housing, and balanced square wave excitation to the bridge is provided. The Wheatstone bridge has resistors in all four arms of the bridge, and at least one of the arms of the Wheatstone bridge has a least two resistive portions with an intermediate tap. One of the probes is connected to this tap, and the other probe may be connected to one of the four terminals of the bridge. The output from the bridge is applied to a differential amplifier and then to a low pass filter to remove residual xe2x80x9cnoisexe2x80x9d arising from the square wave combining function of the differential amplifier. The foregoing circuitry may be mounted in an upper portion of the housing which extends outside the fluid container.
In accordance with one preferred implementation of the invention, the Wheatstone bridge arrangement has first and second input excitation terminals, and first and second output terminals. Fixed resistors may be connected between the first input terminal and the first output terminal and between the first output terminal and the other second input terminal. A fixed resistance may extend between the first of the input terminals and the second output terminal, and two resistors with an intermediate tap may be connected between the second of the input terminals and the second output terminal. Finally, one of the probes may be connected to the tap between the two resistors, and the other probe may be connected to the first input terminal.
The resulting system avoids long term drift, and it stabilized so that it has minimal short term fluctuations.
Concerning the aspects of the construction, a first one of the probes may be mounted at the inner end of the housing, with the other probe being in a ring or sleeve configuration spaced back from and insulated from the first probe. The symmetrical configuration of the probes complements the circular geometry of the threaded housing. The circuitry may be positioned in an outer part of the housing extending outside of the container holding the fluid being measured.
Other objects, features and advantages of this invention will become apparent from a consideration of the following detailed description and from the accompanying drawings.